The invention concerns a process for the preparation of AlCl.sub.3.6H.sub.2 O crystallisate, the thermal decomposition of which provides sufficiently pure alumina for the production of aluminum.
It is known that sufficiently pure AlCl.sub.3.6H.sub.2 O crystallisate may be prepared through multiple crystallizations (German OS No. 1592064). According to this known process, the impure crystallisate derived from the starting solution through evaporation is redissolved in water and a more purified product obtained through multiple evaporations. The redissolution of the crystallisate and the evaporation of the solvent is repeated as many times as necessary to provide a sufficiently pure crystallisate for subsequent use. This multiple recrystallization does allow for the preparation of a sufficiently pure aluminum salt; nevertheless, the evaporation of large quantities of water entails enormous energy and apparatus costs.
A further known method, in which the amounts of water to be evaporated are reduced, involves a multi-step fractional crystallization of the AlCl.sub.3.6H.sub.2 O (German OS No. 1592064, OS No. 1592070). In this known process a sufficiently pure crystallisate is prepared through an incomplete evaporation in the first step. The residual solution enriched in impurities is then passed on to the second step. The impure crystallisate prepared in this step is returned to the first stage and the residual solution further enriched in impurities is evaporated in a third step. The very impure crystallisate is returned to the second stage and the highly impure residual solution is discarded.
This known process does avoid the redissolution of the crystallisate, but requires 3 complete crystallization stages with the associated evaporators, heat exchangers and centrifuges. In addition, two heated stirring tanks are neccessary for the mashing of the crystallisates of the second and third steps. Of the three steps, only the first provides a sufficiently pure crystallisate; the other two steps are used to concentrate the circulating impurities in the minimum amount of discard solution.
A further known method involves the preparation of sufficiently pure AlCl.sub.3.6H.sub.2 O through coarse grain crystallization in a fluidized bed procedure with subsequent washing in a centrifuge with concentrated HCl. (H. O. Poppleton and D. L. Sawyer, Hydrochloric Acid Leaching of Calcined Kaolin to Produce Aluminia, TMS-paper selection, Paper No. A 77-66). This coarse grain crystallization of AlCl.sub.3.6H.sub.2 O is made difficult by the narrow metastable range and the high speed of nucleus formation. Moreover, the rate of falling out of the crystals from the solution is very low on account of the high viscosity of the solution and the minimal density difference between the crystals and the solution. In a fluidized bed process, this leads to a very low flow speed of the supersaturated solution in the fluidized bed, to low yields of crystal per m.sup.3 of supersaturated solution and to low specific crystallization outputs. A further disadvantage of this process is the necessity for recovery of the concentrated hydrochloric acid from the contaminated wash solution.
Still another known process for the preparation of sufficiently pure AlCl.sub.3.6H.sub.2 O crystallisates from impure AlCl.sub.3 solution involves the forced circulation of the crystal suspension through the seeding zone of the crystallizer provided with a sufficient crystal surface for reduction of supersaturation through crystal growth. In this manner is produced a fluid, coarse-grained crystallisate with a low residual moisture content, purification of which may be effected through washing with starting solution from the centrifuge which is poor in impurities. In this known process the supersaturation of the AlCl.sub.3 solution through isothermal evaporation of the solvent is necessary on account of the practically temperature-independent solubility of AlCl.sub.3.6H.sub.2 O. On heat technology grounds this necessitates a multistep crystallization process, in which the heating of the colder steps is principally effected using the vapors generated in the warmer steps, and hot steam is principally used for heating of the warmest steps.